Sure, nobody really thinks of worm sperm as being cute and cuddly -- assuming one thinks about them at all -- but who would have thought they were stone cold killers?

In a paper published Tuesday in PLOS Biology, a team of researchers who apparently enjoy watching nematodes get jiggy were shocked to learn that male sperm will conduct search-and-destroy missions on females and hermaphrodites of other worm species.

Instead of simply fertilizing the partner's eggs and being done with the whole exercise, the sperm of male Caenorhabditis nematodes who mate with other species will not only bully "milder" sperm and cause sterilization, they'll also destroy body tissues like a metastasizing cancer, authors say.

"Our findings were quite surprising," co-author Asher Cutter, an evolutionary biologist at the University of Toronto, said in a statement.

The study provides new insight into assortative mating -- the tendency for animals to reproduce with similar creatures -- as well as the evolutionary pressures posed by "aggressive sperm."

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"Punishing cross-species mating by sterility or death would be a powerful evolutionary way to maintain a species barrier," senior study author Eric Haag, an evolutionary biologist at the University of Maryland, said in a statement.

Animals of the same genus but different species may mate and produce offspring, but that animal is usually sterile. An example is the mule -- which occurs when you breed a horse and a donkey.

The case of the worms, however, is more drastic.

Researchers mixed and matched three Caenorhabditis species, cross-breeding male, female and hermaphrodite worms -- worms capable of producing both sperm and eggs in order to reproduce.

The males, researchers said, mated indiscriminately when given the opportunity. The hermaphrodites however would attempt to avoid copulation by crawling away from the breeding area when they sensed a male of another species.

Study authors learned why this was the case when they examined sterile and dying worms under a microscope. Fluorescent stains showed that once the male sperm had entered the hermaphrodite worm, they overpowered the hermaphrodite's smaller sperm and blocked ovulation.

After that, things just got worse. The aggressive sperm invaded the worm's ovary, and caused its eggs to mature prematurely. The sperm then traveled deeper into the worm's body, causing tissue damage and, eventually, death.

Authors wrote that while hermaphrodite worms suffered the most in cross-species breeding, damage was also done to female worms in the form of compromised reproduction and reduced longevity.

Study authors surmised that the aggressiveness of the males' sperm was the result of competition with other male worms of the same species. When females worms mated with multiple males, their sperm were obliged to jostle for superiority within the female's reproductive tract.

The authors described this development of ever stronger sperm as "an evolutionary intra-species arms race."

Whether or not these aggressive sperm were also intended to hamper the reproduction of other species is unclear, authors said.

"It remains to be tested whether ... sterilization by sperm could also be co-opted as a weapon in inter-species resource competition when multiple Caenorhabditis species inhabit the same resource patch."